This invention relates generally to pultruding braids to form composites.
Pultrusion is a fabrication method that is often used for producing elongated rigid tubular structures. Generally, pultrusion is defined as a process for producing reinforced composites by pulling a resin impregnated fiber substrate through a forming and curing die. Pultrusion has its origins in the early 1950s when it was used to form the pole sections for fishing rods. The methods and machinery for forming pultruded parts remained the same until the late 1960s when improvements allowed manufacturers to form various structural shapes used to produce ladders, handrails, walkway supports and the like. Pultrusion is used today to produce hollow and solid reinforced composites of various dimensions.
One known form of pultrusion utilizes tubular braids made by a braiding machine as part of the processing line to form tubular composites. It includes mounting a braiding machine at the beginning of the pultrusion process to produce the braid. Individual axial fibers are passed through a matrix resin and are fed through the braiding machine where they meet bias fibers and are braided to form a braid. The braid is then pulled from the braiding machine to a pultrusion die where it is heated and formed in order to produce a rigid composite. In other processes having a braiding machine on the processing line, the braid is formed on the braiding machine and is then fed through a resin injection component (e.g., a bath) followed by a pultrusion component. Braiders which are incorporated in pultrusion processes are referred to as in-line braiding machines. Such in-line braiding machines are shown in U.S. Pat. No. 5,468,327 to Pawlowicz et al. (hereinafter, the ""327 patent), the contents of which are incorporated herein by reference. However, there are several disadvantages with using in-line braiding machines in conjunction with pultrusion. One disadvantage is that in-line braiders are typically small machines (e.g., with 144 or fewer carriers) due to the physical difficulties of side-mounting large machines as well as the added cost of building and maintaining large, side-mounted braiders. As a result, in-line braiding can limit the size (i.e., diameter) of the braiding machine which can be used in a pultrusion process. Consequently, the braid is similarly limited in diameter.
Another disadvantage is that producing braided substrates using an in-line braiding machine limits the process to the speed of braiding considerations in addition to pultrusion considerations. The braiding machine speed is based on many factors: yarn spool size dictates the need to stop and start the machine in order to splice in new pieces; and, splicing takes manual intervention, which ultimately stops the pultrusion process using an in-line braiding machine. In addition, there are also numerous quality checks that need to be made prior to braid progressing to the pultrusion process. Braid needs to be checked for imperfections and inconsistencies before consideration for downstream processing. These factors make in-line braiding machines slower than the pultrusion process.
Accordingly, the manufacture of braid is optimally separated from downstream processing, such as pultrusion. For example, a braid can be manufactured on a braiding machine and subsequently wrapped around a storage reel for later use in a pultrusion process. Braids removed from the braiding machine on which they were produced prior to use in further processing are hereinafter referred to as pre-consolidated braids. In addition, pre-consolidated braids have further advantages over in-line braids for use in pultrusion. Pre-consolidated braids offer flexibility in braid choice for a single pultrusion set-up. For example, a one inch diameter braid can be made by 15 different sizes of braiding machines, each of which yield a different wall thickness, fiber angle and raw material. Each of these braid styles can be accommodated by one mandrel in the pultrusion process. Yarn choice is also expanded with pre-consolidated braid because the conditions for braiding some materials (e.g., braiding in a dust free environment etc.) are better met outside of the pultrusion process.
Another known form of pultrusion utilizes flat substrates, including a flat braid or a flat non-braided material with fibers oriented therein, to form tubular composites. The flat braided substrates generally are pre-consolidated. The pultrusion process includes passing the substrate through a matrix resin bath. The wetted substrate is then pulled through a die having a mandrel located in the center of the die for forming purposes. Upon entry into the die, the substrate must pass a bolted fixture that holds the mandrel in place. A bolted fixture is needed in order to keep the mandrel stationery and concentric. Such mandrel orientation is desired to provide an even coating of resin around the braid. After passing the bolted fixture, the flat substrate is wrapped around the mandrel so that its ends (along its longitudinal axis) are connected to form a shape inside the die. One exemplary shape is a hollow tube. The formed substrate is heated to produce a rigid composite structure having a predetermined shape. However, the use of tubular braids for forming tubular composites has advantages compared to wrapped flat substrates. Tubular braids have continuous interlocking fibers which have uniform material distribution, no overlapping material and improved hoop strength (also known as transverse strength) when converted into a composite.
Therefore, there is a need for pultruding pre-consolidated tubular braids. However, there are barriers to pultruding such braids because the pultrusion components used for in-line braids or pre-consolidated flat braids are not conducive for use with pre-consolidated tubular braids. Such known components are pultrusion dies, mandrels and resin injection devices. One barrier is that the braid must be supported by a mandrel along the processing line. For in-line braids, the mandrel which forms part of the braiding machine supports the braid. For example, in the ""327 patent, an in-line braiding machine (FIG. 1) is shown. The mandrel which forms part of the in-line braiding machine supports the braid throughout the subsequent process. The mandrel is supported as a cantilever upstream of the machine by a beam, which is supported by the base. Therefore, the beam supports the mandrel.
Where pre-consolidated flat braids are used, the mandrel is held in place using one or more bolted fixtures. A flat braid is then drawn over the bolts and thereafter wrapped (along its longitudinal axis) to form a tubular shape for curing, as described above. However, the bolts holding the mandrel in place would prevent pre-consolidated tubular braids from advancing over the mandrel. For example, in the ""327 patent, a pre-consolidated tubular braid would be prevented from being pulled over the mandrel due to the beam.
Another barrier to using pre-consolidated tubular braids is that the mandrel should have a concentric orientation to the extent possible in the resin injection chamber and pultrusion die. Such concentric orientation is desired in order to provide an even coating of resin around the braid and to minimize the load requirements for pulling the braid through the pultrusion process. Otherwise, the wall thickness of the cured braid will be uneven. This, in turn, reduces the strength of the composite. Where the braid is in-line, the mandrel is supported as part of the braiding machine. Where the braid is a pre-consolidated flat substrate, support can be provided by bolting the mandrel to the processing line. However, such bolting is not possible with pre-consolidated tubular braids. Therefore, if the bolts are removed, there is no support for maintaining a concentric orientation or for holding the mandrel in place. Therefore, known mandrels do not support pre-consolidated tubular braids in pultrusion.
A further barrier to using pre-consolidated tubular braids is that composite materials formed with pultrusion require a thorough impregnation of the fiber substrate in order to deliver optimal composite performance. Resin baths are often used in pultrusion in order to mingle the matrix resin with the fibers of braids or flat surfaces of the substrates. These resin baths, however, do not adequately force the matrix resin through the fibers, and therefore are not adequate for pre-consolidated tubular braids since the tubular shape of braid guards its interior. Also, the flat surface of most mandrels prevents resin from thoroughly impregnating the braid. Pressurized resin chambers are also available. However, they wet-out only sections of braid and are not built to handle the continuous nature of pre-consolidated tubular braid. For example, in U.S. Pat. No. 5,407,616 to Dube, there is shown a resin injection chamber for impregnating a braid with resin. However, the braid and mandrel in combination are moved through the chamber. Therefore, the length of the braid is limited to the length of the mandrel. In addition, the process is not continuous because a single braid/mandrel combination is processed at a time. This patent is incorporated in its entirety herein by reference.
In summary, there is a need for pultrusion process components which enable the use of pre-consolidated tubular braids to form tubular composites. However, known pultrusion processes do not adequately provide a mandrel which supports pre-consolidated tubular braids and is maintained in a concentric orientation nor provide a resin injection component which thoroughly impregnates pre-consolidated tubular braids.
It is an objective of our present invention to provide a new pultrusion method and device for forming tubular composites utilizing pre-consolidated tubular braids alone or with additional materials such as fibers, fabrics, mats, tapes or other similar materials. It is another objective of our present invention to adequately impregnate matrix resin into pre-consolidated tubular braids.
One embodiment of our invention is directed to providing a mandrel which supports pre-consolidated tubular braids and is maintained in a concentric orientation in the resin injection chamber and pultrusion die. The supportive mandrel can include a xe2x80x9cSxe2x80x9d shape section over which a pre-consolidated tubular braid is pulled. The xe2x80x9cSxe2x80x9d shaped mandrel is oriented within roller drives to support the mandrel and assist in moving the braid. The mandrel can also include centering components, in the form of turnbuckles, before or after the resin injection chamber or pultrusion die. The mandrel is maintained in a concentric orientation with the resin injection chamber and pultrusion die by the support given to it from the roller drives in the S shape section as well as the centering components. The S shape section holds the mandrel in place along its longitudinal axis in response to the forces due to movement of the braid over it. In addition, the tension placed on the braid as it is pulled through the process causes it to seek the center of the chamber and die. The centering components enable the mandrel to move along the mandrel""s radial axes just before the chamber or die in order to support the braid""s radial movement toward the center. Consequently, the concentric orientation of the mandrel inside the chamber and die is optimized.
A second embodiment of our invention is directed to providing a pressurized resin injection chamber with a mandrel which provides adequate impregnation of pre-consolidated tubular braids. The pressurized resin injection chamber includes a portion of the mandrel with depressions on its surface, for example, down its length or forming a helical winding down its length etc., such that resin flows through the surface of the pre-consolidated tubular braid, past the braid fibers located on the braid""s interior and into the depressions. Then, the resin can flow within the depressions along the length of the mandrel and exit the braid by contacting the braid fibers located in the braid""s interior and then flowing through the braid to its surface.
Our invention provides the advantage that pre-consolidated tubular braids can now be adequately pultruded. In addition, pre-consolidated tubular braids which are pultruded in order to produce a composite have the following advantages compared to in-line braids: the size of the braid (as determined by the size of the braiding machine) is not limited to small machines (e.g., typically with 144 on fewer carriers) and the speed of the pultrusion process is not limited to the speed of the braiding machine. In addition, pre-consolidated tubular braids have numerous advantages compared to a wrapped flat pre-consolidated braids or flat non-braided substrates (as are used in known pultrusion processes). Pre-consolidated tubular braids have continuous interlocking fibers which have uniform material distribution, no overlapping material and improved hoop strength when converted into a composite. In addition, since the concentric orientation of the mandrel is optimized, the cured braid""s walls are evenly coated with resin. Moreover, the pre-consolidated braid is adequately wetted-out to provide more intermingling between the braid and matrix resin, which ultimately provides better strength properties in the end product composite.